1. Field of the Invention
This invention relates to the combustion at a burner tip of at least one waste gas stream containing at least one unsaturated or aromatic hydrocarbon that forms coke at the temperature of combustion of the waste gas stream.
More particularly, this invention relates to the reduction of coke fouling of a burner tip when combusting a waste gas stream containing at least one coke forming unsaturated or aromatic hydrocarbon.
2. Description of the Prior Art
Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes.
An olefin production plant is generally composed of a cracking unit and a hydrocarbons purification unit.
In the cracking unit a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil or natural gas liquids is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated. This mixture, after preheating, is subjected to hydrocarbon thermal cracking using elevated temperatures (1,400 to 1,550 degrees Fahrenheit or F.) in a pyrolysis furnace. This thermal cracking is carried out without the aid of any catalyst.
The cracked product effluent of the pyrolysis furnace (furnace) contains hot, gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule, or C1 to C35 inclusive, both saturated and unsaturated). This product contains aliphatics (alkanes and alkenes), alicyclics (cyclanes, cyclenes, and cyclodienes), aromatics, and hydrogen.
This furnace product is then subjected to further processing in the cracking unit to produce, as products of the olefin plant, various, separate and individual product streams such as hydrogen, ethylene, propylene, fuel oil, and pyrolysis gasoline. After the separation of these individual streams, the remaining cracked product contains essentially C4 hydrocarbons and heavier. This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while a C5 and heavier stream is removed as a bottoms product.
Pyrolysis furnaces are typically fired with a fuel gas that passes through a plurality of burner tips disposed with burners near the bottom of the pyrolysis furnace. In addition, the plant's fuel gas is burned in various boilers needed for the purification of the olefin and aromatic products. Besides olefin units, such fuel gas burners are used in various other chemical and refining units. This invention applies to all process units using fuel gas burner tips.
A desired fuel gas is natural gas or other fuel gases which are free of unsaturated hydrocarbons. However, from time to time it is necessary or desirable to burn in such furnaces waste gas streams generated elsewhere in the plant or other hydrocarbon processing facilities such as refineries. Substantial amounts of olefin or aromatic hydrocarbons may exist in such waste gas streams.
The temperature of combustion of the fuel gas at the burner tips in such pyrolysis furnaces can reach or exceed 2,000 F, and in so doing can form from unsaturated or polyaromatic hydrocarbons in the fuel gas stream a variety of coke species such as thermal coke, catalytic coke, carbidic and graphite coke and the like. These coke species can deposit on and in the burner tip, particularly low NOx burner tips.
Waste gas streams containing unsaturated hydrocarbons such as olefins (mono-olefins and/or poly-olefins) and aromatics tend to form coke on the burner tips at the combustion temperatures at which those burner tips operate. Coke formation on the burner tips fouls the burner tips thereby reducing their efficiency of combustion, and, ultimately, plugging the tips altogether.
When employing fouling waste gas streams which form coke at the burner tip, it is currently necessary to mix these streams with non-fouling (no unsaturated/aromatic hydrocarbon content) fuel gas to minimize the tendency to form coke at the burner tip. This degrades the combustibility characteristics of the clean fuel gas, and does not altogether eliminate the formation of coke at the burner tip.
It is desirable to be able to use fouling waste gas streams more fully since they are plentiful and low cost, and to do so without resorting to mixing with non-fouling, high grade fuel gas streams. In addition these non-fouling fuel gases typically are of a higher value, and, therefore, cost more per BTU of heat generated.